Jesus’ concept of Gehenna must be viewed against the background of ancient rabbinic perspectives, which are nicely summarized with copious documentation in the excellent Anchor Bible Dictionary article [Vol. 2]: “Most of those [Jews] who enter it [Gehenna] in the intermediate state would be released from it…It was a fiery purgatory for those Jews whose merits and transgressions balanced one another who would afterward be admitted to Paradise. Often the punishment of Gehenna was restricted to 12 months.”
Jesus’ parable of the unforgiving servant uses a debtor’s prison as an image for the limited duration of punishment in Gehenna: “And in his anger his lord handed him over to be tortured until he would pay his entire debt (Matthew 18:34).” Here the debt’s payment and ultimate release remain a possibility. How the debt might be paid off remains unclear, but expiation and purgation remain possibilities. Remember, “debt” (Aramaic: “chob”) is the Aramaic term for “sin” that inspires this image of Hell as a debtor’s prison.
More controversial is the related possibility that Matthew 5:25-26 refers to Gehenna:
“Make friends quickly with your accuser while you are on the way to court with him, or your accuser may hand you over to the Judge, and the Judge to the guard, and you will be thrown into prison. Truly I tell you, you will never get out until you pay the last penny.”
I prefer the symbolic interpretation of the prison as Gehenna for 4 reasons:
(1) This saying makes little sense if taken literally. Jesus would in effect be saying: “Let me tell you how to beat the rap of criminal charges. Wait till you and your accuser are actually on the road on the way to court and then kiss up to him.”
(2) "Jesus always applies the formula “Truly I say to you” to our relationship with God, never to a purely secular issue like a court proceeding.
(3) In the first 2 centuries this saying is always interpreted symbolically.
(4) In the Lukan context (12:57-59) the saying is located in an eschatological context.
That said, Matthew 18:34 removes the necessity of invoking this saying to establish Jesus’ image of Gehenna as a debtor’s prison.
Jesus’ image of “few stripes” as an image of punishment in Gehenna implies a finite limit and therefore ultimate release: “The slave who did not know and did what deserved a beating will be beaten with few stripes (Luke 12:48).”
As for Jesus’ view that people who don’t follow Him can be saved, Mark 9:40 is certainly relevant: “Whoever is not against us is for us.”